1. Field of the Invention
The present invention is directed to a compact interferometer allowing operation at normal incidence while outputting one or more periodic signals to be monitored, e.g., by a wavelength monitor. In particular, this invention is directed to suppressing higher order modes of interference while allowing the interferometer to be positioned at normal incidence.
2. Description of Related Art
The use of a shaped etalon to generate periodic signals is disclosed, for example, in commonly assigned, co-pending PCT Application Serial No. PCT/US01/21272 entitled “Integrated Wavelength Locker for use with more than one wavelength and Associated Methods” filed Jul. 6, 2001, the entire contents of which are hereby incorporated by reference for all purposes. The periodic signals are processed in accordance with phase shifting interferometry techniques to determine a wavelength of the light being monitored. Alternatively or additionally, a reference signal may be used in conjunction with one or more periodic signals to determine the wavelength.
In optimizing the sinusoidal nature of the periodic signals, suppression of higher order etalon modes is desired. This is typically achieved by providing the shaped etalon at an angle, e.g., 15 degrees. This tilt is greater than the amount of tilt incorporated to reduce back reflections from the etalon, which is typically on the order of a few degrees. However, mounting the etalon at a tilt can be difficult. Further, this tilt takes up space along the optical axis of the system, which is typically at a premium. Finally, the tilt reduces the contrast of the signals.